1. Field of the Invention
The invention relates to pistons and in particular to pistons for internal combustion engines or compressors.
2. Review of the Prior Art
A typical piston of a known design has a crown, a ring band, two gudgeon pin bosses defining a gudgeon pin bore and a skirt depending from the ring band and extending around the piston. The function of the skirt is to guide the piston in its reciprocating movement in an associated cylinder or liner. As well as performing this reciprocating movement, the piston also moves laterally within the associated cylinder or liner as a result of the clearance needed between the piston and the associated cylinder or liner and as a result of the varying direction of the forces applied to the piston by an associated connecting rod.
These movements can result in so-called "piston slap" which radiates sound energy that is noticeable and intrusive above the general engine noise. This noise can occur on starting, during acceleration of the engine or at high speeds or loads, or during two or more of these conditions. The slap can occur at any part of the piston cycle, although most radiated noise is usually produced just after top dead centre firing when the rapid increase in combustion cylinder pressure causes large lateral accelerations of the piston and thus high impact velocities between the piston and the associated cylinder or liner.
There have been many proposals for reducing the noise generated in this way. Amongst these have been proposals, in pistons for internal combustion engines, directed to the reduction of heat flow from the crown to the skirt to allow the piston, when cold, to be a closer fit in the associated cylinder or liner and so decrease the scope for lateral movement of the piston. A similar effect in such pistons has been sought by the use of expansion control inserts which limit the expansion of the piston on heating, so again allowing a tighter initial fit.